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Abstract

Selected mineralized black shales of Devonian age from the Selwyn Basin, Northwest Territories (Canada) were analyzed by Nuclear Reaction Analyses (NRA) and electron microprobe for nitrogen and carbon in silicates, sulfides, phosphates and organic matter in order to give new insights on nitrogen and carbon fractionation processes during diagenesis and hydrothermal infiltration. Hydrothermal feldspars show tri-modal composition: albite, high nitrogen-bearing K-feldspar (56 mol% buddingtonite (NH4AlSi3O8·1/2H2O, hydrous ammonium-feldspar, 51 mol% orthoclase) and hyalophane (32 mol% celsian). Barium-rich feldspars (hyalophane) contain lowest nitrogen contents. Potassium and nitrogen are positively correlated, while nitrogen and barium are negatively correlated due to the replacement of monovalent NH4+ by divalent Ba2+. The Ba-rich K-feldspar rim shows penetrative textures towards an internal K–N-rich core that is interpreted as diffusive overgrowth. These feldspars are interpreted to be deposited from hot hydrothermal Ba-bearing fluids. The second important nitrogen carrier is organic matter (from 0.6 to 0.66 wt.%). Hydrothermal quartz (N=527 ppm), diagenetic biogenic F-rich apatite (conodonts: N=468 ppm,), biogenic Fe–Ni sulfides (N=380–620 ppm) and abiogenic Ni–Fe sulfides (N>440 ppm) contain homogeneously distributed nitrogen with amounts 10-fold lower than those measured in organic matter. A two-step nitrogen-release model is suggested to explain the nitrogen-partitioning in these minerals. Primary organic matter breakdown is considered to liberate nitrogen, phosphate and sulfur to pore fluids and the water column, providing nutrients for vent fauna growth. Sulfurization, due to microbial sulfate reduction, and silicification of the vent fauna releases nitrogen in a second step. Minor nitrogen was trapped as organic molecules in conodonts, while the majority was transported by hydrothermal fluids and was incorporated as ammonium in feldspars substituting for potassium.